1 . Field of the Invention
The present invention relates to films based upon or containing oriented dye. It also relates to methods of manufacturing such films and to polarizers and liquid crystal display devices utilizing the films. More specifically, the invention pertains to films made of or containing highly oriented dichroic dye, methods of manufacturing such films, and high-performance polarizers and liquid crystal display devices utilizing the film.
2. Description of Related Art
Polarizers are used as elements of liquid crystal displays (hereinafter referred to as LCDs). Polarizers have the property of absorbing some of the light transmitted from the LCDs. This becomes a particularly acute problem with full color LCD""s since full color LCD""s contain color filters as well. The polarizer present in the LCD and the color filter used in the full-color LCDs both absorb light. This reduces the quantity of transmitted light to a level that is often quite low. A powerful backlight is thereby required for sufficiently bright LCDs. This backlight can undesirably raise the temperature of the liquid crystal cell and increase the power consumed. One type of polarizer is manufactured by adsorbing iodine onto a uniaxially stretched, oriented alignment film of polyvinyl alcohol (hereinafter referred to as PVA) or a derivative of PVA.
A second type of polarizer is based on dichroic dyes and is similarly produced by adsorbing the dye onto a uniaxially stretched oriented PVA or PVA derivative alignment film. (Throughout this specification and claims, the terms xe2x80x9cdichroicxe2x80x9d and xe2x80x9cdichroic dyexe2x80x9d have their commonly accepted meanings and refer to the property of presenting different optical properties e.g., colorxe2x80x94when viewed from different directions, and thus to in effect give a polarized response.)
A third type of polarizer is produced by uniaxially stretching a polyvinyl chloride or PVA film and then generating polyene units as chromophores by dehydrohalogenation of the polyvinyl chloride or by dehydration of the PYA.
The iodine-containing polarizers initially exhibit excellent polarizing power. These polarizers, however, have poor resistance to water and heat and are not sufficiently durable under conditions of high temperature and high humidity. Protective films or other possible measures typically do not significantly improve the durability of the iodine-containing polarizers.
The dichroic dye polarizers have greater resistance to water and heat than the iodine-based materials, but have inadequate polarizing properties for most commercial applications. Pigments, which have excellent weather resistance, cannot be substituted since they are not highly oriented in polymer films and thus cannot be used for polarizers.
In addition, many difficulties arise when trying to manufacture fine polarizers such as polarizers having a width of not greater than about 200 micrometers. Cutting a dyed alignment film is the only way presently used to prepare such fine polarizers.
Thus, the known methods of orienting dye molecules include the use of an oriented PVA film which aligns the dye molecules as has been described above. It is also known that one can mix a dye with a liquid crystal material and orient the dye as the liquid crystal molecules in the liquid crystal cell are themselves aligned. In the latter method, a liquid crystal material must be sealed between two glass plates or placed within a cell. This method further requires that an alignment film be formed on the surface of the glass plates. The degree of alignment of the dye depends upon the degree of the alignment of the liquid crystal material, so that in many cases the polarizer does not have a sufficient polarizing power due to low alignment of the liquid crystal material.
Films containing highly oriented dichroic dyes and particularly films which include micropatterned, highly oriented dichroic dyes and their easy manufacture are thus in high demand.
Although not relating to orienting dye molecules, J. C. Wittmann et al. have found that an oriented polytetrafluoroethylene (hereinafter referred to as PTFE) tin film may be formed by rubbing PTFE while applying heat and pressure. They have reported that alkanes, liquid crystal molecules, polymers, oligomers, and inorganic salts can be oriented using this oriented PTFE film as an alignment layer (See Nature, Vol. 352, p. 414 (1991) and U.S. Pat. No. 5,180,470). While alignment of these organic molecules with this oriented PTFE thin film has been reported in detail, alignment of dye molecules useful for polarizers, or more specifically, alignment of dichroic dye molecules for polarizers, has not been elucidated.
It has now been found that through the use of fluororesin orientation films such as PTFE orientation films, one can achieve highly oriented dichroic dyes. The present invention concerns these oriented dye-containing films, methods of manufacturing these films; and high performance polarizers and liquid crystal display devices which utilize these films.
Thus, in one aspect, this invention provides a uniaxially oriented film which includes one or more dichroic dyes. This film contains at least 1% by weight dichroic dye up through being completely dichroic dye (100% by weight dye). The remainder is a binder polymer or diluent or the like. The film has a thickness of from 1 nanometer to 5 micrometers, a wavelength of peak absorbance of from 400 to 800 nanometers, and a dichroic absorbance ratio of not less than 25 at the wavelength of peak absorbance.
In another aspect, this invention provides a body of micro-patterned dye-containing film. This film comprises at least 1% by weight up to 100% by weight of uniaxially oriented dichroic dye. This body of film has a thickness of from 1 nanometer to 5 micrometers and a wavelength of peak absorbance of from 400 to 800 nanometers and a dichroic absorbance ratio of not less than 10 at the wavelength of peak absorbance. This body has a length and width in the film plane with the width being in the range of from about 1 to about 200 micrometers.
In another aspect, this invention provides a method for manufacturing the uniaxially oriented film just described. This method involves forming a film from a film-forming material which includes at least 1% by weight dichroic dye and has a thickness of from 1 nanometer to 5 micrometers and a wavelength of peak absorbance of from 400 to 800 nanometers. This film-forming takes place on a substrate having a fluororesin alignment layer as its surface. This produces a uniaxially oriented film having a dichroic absorbance ratio of not less than 25 at the wavelength of peak absorbance. The film-forming material may in some cases be neat dichroic dye but alternatively may include a film-forming binder polymer mixed with dichroic dye. In more specific aspects, this method may include additional steps of stripping the film from the forming substrate and applying the film to a supporting substrate different from the forming substrate.
In yet an additional aspect, this method may be used to manufacture bodies of micropatterned dye-containing film. In this embodiment, the alignment substrate should have a defined width in the film plain of from about 1 to about 200 micrometers with the fluororesin alignment layer being present on the surface in that defined-width region. This will yield bodies of uniaxially oriented film having a defined width of from about 1 to about 200 micrometers and a dichroic absorbance ratio of not less than 10 at the wavelength of peak absorbance. As with the more general method, this method may include the additional steps of stripping the film from the forming substrate and applying the film to a supporting substrate.
In another aspect, this invention provides polarizers. A polarizer results when a transparent support having a planar surface has a uniaxially oriented film as described above attached to its planar surface. Typical transparent supports for such polarizers can be polymer films, glass supports or transparent electrodes.
In an additional aspect, the invention can provide micropolarizers having the same general structure. These micropolarizers include the same type of transparent support carrying a body of the micropatterned dye-containing uniaxially oriented film upon its planar surface.
In additional aspects, this invention provides liquid crystal display devices. These devices include a liquid crystal cell containing a pair of separated transparent substrates having electrodes thereon and a nematic liquid crystal layer positioned between the substrates. The nematic liquid crystal layer has a positive dielectric anisotropy and a helical axis aligned vertically to the substrates and oriented substantially horizontally with a twist angle from 90 to 270 degrees when no voltage is applied. In the display devices of the invention, a uniaxially oriented film or micropatterned dye-containing film or polarizer or micropolarizer as just described is positioned in the path of viewing the LCD. In some embodiments, the uniaxially oriented material is placed outside of and adjacent to the liquid. crystal cell. The uniaxially oriented films may be positioned on the transparent substrates of the liquid crystal cell as well.